In fuel cells, water is produced by a cell reaction as a product in principle. Fuel cells have therefore drawn attention as clean power generation systems without a substantially harmful influence on the earth's environment. For example, a polymer electrolyte fuel cell comprising a pair of electrodes on both sides of a polymer electrolyte membrane that conducts protons produces electromotive force by supplying hydrogen gas as a fuel gas to one of the electrodes (i.e., the fuel electrode: anode), and supplying oxygen gas or air as an oxidant to the other electrode (i.e., the air electrode: cathode).
The cell characteristics of polymer electrolyte fuel cells have been drastically improved by advances such as the following: (1) a polymer electrolyte membrane having high ion conductivity has been developed; and (2) catalyst-carrying carbon coated with an ion-exchange resin (the polymer electrolyte) consisting of a material that is the same as or different from that of the polymer electrolyte membrane is used as the constituent material of the electrode catalyst layer to form what is called a three-dimensional reaction site in the catalyst layer. In addition to the excellent cell characteristics described above, the polymer electrolyte fuel cell can readily be made smaller and lighter. Due to the characteristics described above, the polymer electrolyte fuel cell is expected to be put in practical use as a power source for mobile vehicles such as electrically powered cars or power sources for small cogeneration systems.
In general, the gas diffusion electrode used in a polymer electrolyte fuel cell consists of a catalyst layer, which contains catalyst-carrying carbon materials coated with said ion-exchange resin, and a gas diffusion layer, which not only supplies the reaction gas to the catalyst layer but also collects electrons. The catalyst layer has open areas consisting of micropores formed among secondary or tertiary carbon particles, which are constituents of the catalyst layer, and the open areas function as diffusion channels of the reaction gas. As such catalysts, noble metal catalysts, such as platinum or platinum alloy, that are stable in ion-exchange resin are generally used.
In the past, a polymer electrolyte fuel cell involved the use of catalysts comprising a noble metal, such as platinum or platinum alloy, carried on carbon black as cathode and anode catalysts of the electrode catalysts. In general, platinum-carrying carbon black is prepared by adding sodium bisulfate to an aqueous solution of platinic chloride, allowing the mixture to react with a hydrogen peroxide solution, preparing the carbon black particles to carry the resulting platinum colloids, washing the resultants, and heat-treating the resultants as needed. Electrodes of a polymer electrolyte fuel cell are prepared by dispersing platinum-carrying carbon black particles in a polymer electrolyte solution to prepare an ink, coating the gas diffusion substrate, such as a carbon paper, with the ink, and drying the substrate. The polymer electrolyte membrane is sandwiched between such two electrodes, followed by a hot press. Thus, a membrane electrode assembly (MEA) can be constructed.
Platinum is an expensive noble metal, and it is thus expected to exhibit satisfactory performance by a small amount thereof. Accordingly, work is proceeding with catalyst activity in smaller amounts of platinum. For example, JP Patent Publication (kokai) No. 2002-289208 (A) is intended to provide an electrode catalyst for a fuel cell having high durability by inhibiting growth of platinum particles during operation, and discloses an electrode catalyst comprising a conductive carbon material, metal particles carried thereon that are less likely to be oxidized than platinum under acidic conditions, and platinum covering the outer surface of the metal particles. Specifically, the publication exemplifies an alloy comprising platinum and at least one metal selected from among gold, chromium, iron, nickel, cobalt, titanium, vanadium, copper and manganese as the metal particle.
JP Patent Publication (kokai) No. 2002-15744 (A) is intended to provide a polymer fuel cell that has excellent cathode polarization properties and produces a high cell output, and disclose the catalyst layer of the cathode containing a metal catalyst selected from the group consisting of platinum and platinum alloy and a metal complex containing a given amount of iron or chromium to improve cathode polarization properties. Specifically, in a polymer electrolyte fuel cell in which the cathode comprises a gas diffusion layer and a catalyst layer located between the gas diffusion layer and the polymer electrolyte layer, the catalyst layer contains a noble metal catalyst selected from the group consisting of platinum and platinum alloy and a metal complex containing iron or chromium, and the content of the metal complex is 1 to 40 mol % of the combined quantity of the metal complex and the noble metal catalyst. Thus, the metal complex containing iron or chromium in the catalyst layer of the cathode can effectively reduce the overvoltage activated by the oxygen reduction reaction of the cathode. Consequently, the cathode polarization properties can be improved, and high cell output can be attained.
In a polymer electrolyte fuel cell, peroxides are generated in the catalyst layer formed at the interface between a polymer electrolyte membrane and an electrode through a cell reaction, and the generated peroxides are diffused and converted into peroxide radicals, which cause electrolytes to deteriorate. For example, a fuel is oxidized at the fuel electrode, and oxygen is reduced at the oxygen electrode in a fuel cell. When hydrogen is used as a fuel and an acidic electrolyte is used, an ideal reaction is represented by the following formulae (1) and (2).Anode (hydrogen electrode): H2→2H++2e−  (1)Cathode (oxygen electrode): 2H++2e−+(½)O2→H2O  (2)
The hydrogen ions generated at the anode by the reaction represented by formula (1) permeate (diffuse through) the polymer electrolyte membrane in a hydrate state of H+ (XH2O), and the hydrogen ions that permeate the membrane are then subjected to the reaction represented by formula (2) at the cathode. The electrode reactions at the anode and at the cathode involve the use of the electrode catalyst layer that is in close contact with the polymer electrolyte membrane as the reaction site, and such reactions proceed at the interface between the catalyst in the electrode catalyst layer and the polymer electrolyte membrane.
In addition to the main reactions, however, side reactions take place in a real fuel cell. A typical example thereof is generation of hydrogen peroxide (H2O2). Although the mechanism thereof is not fully understood, a possible mechanism is as follows. That is, hydrogen peroxide can be generated at the hydrogen or oxygen electrode. At the oxygen electrode, for example, hydrogen peroxide is deduced to be generated in a manner represented by the following formula (3) through the incomplete reduction of oxygen.O2+2H++2e−→2H2O2  (3)
Oxygen that is contained as a contaminant or intentionally included in gas at the hydrogen electrode or oxygen that is dissolved in an electrolyte at the oxygen electrode and diffused through the hydrogen electrode is considered to be associated with the reaction, and such reaction is considered to be represented by a formula identical to said formula (3) or the following formula (4):2M−H+O2−→2M+H2O2  (4)
wherein M is a catalyst metal used for the hydrogen electrode, and M-H is the catalyst metal trapping hydrogen. In general, a noble metal such as platinum (Pt) is used for a catalyst metal.
Hydrogen peroxide that is generated on such electrodes separates from the electrodes by diffusion or other means and migrates into an electrolyte. Hydrogen peroxide is a substance having a potent oxidizing power and oxidizes many types of organic matter that constitute an electrolyte. Although the detailed mechanism thereof has not been elucidated, it is often considered that hydrogen peroxide produces hydroxyl radicals, and such hydroxyl radicals function as direct reactants for oxidization. Specifically, the radicals generated by the reaction by such as following formula are considered to abstract hydrogen from the organic matter of an electrolyte or cleave another bond. The cause for radical generation has not yet been elucidated; however, contact with a heavy-metal ion is considered to result in catalyst activity. Further, radical generation is considered to be caused by heat, light, or the like.H2O2→2.OHorH2O2→.H+.OOH
JP Patent Publication (kokai) No. 2001-118591 (A) discloses a technique to solve the abovementioned problem, wherein the technique can prevent deterioration of a fuel cell caused by radicals by adding a compound, which “degrades,” “inactivates” and “traps and inactivates” radicals generated by permeated hydrogen, to the electrolyte. More specifically, transition metal oxides, such as manganese oxides, ruthenium oxides, cobalt oxides, nickel oxides, chromium oxides, iridium oxides or lead oxides, which catalytically degrade peroxides, are dispersed and incorporated into a polymer electrolyte. Alternatively, a stabilizer for peroxide, such as a tin compound, that inhibits generation of peroxide radicals may be dispersedly incorporated therein. Further, a compound having a phenolic hydroxyl group that traps and inactivates the generated peroxide radicals may be incorporated.
As mentioned in JP Patent Publication (kokai) No. 2002-289208 (A), when a noble metal/base metal alloy catalyst is used, a base metal, such as iron, that is a counterpart member material of a noble metal such as platinum is eluted during the use of a fuel cell, and it disadvantageously causes the durability of a fuel cell to deteriorate as a contaminant of an electrolyte.
As JP Patent Publication (kokai) No. 2002-15744 (A) discloses, when a metal complex having iron or chromium is used as a promoter, a high cell output can be attained at the initial stage. However, iron or chromium is eluted during the use of a fuel cell, and it disadvantageously causes the durability of a fuel cell to deteriorate as a contaminant of an electrolyte.
In the method disclosed in JP Patent Publication (kokai) No. 2001-118591 (A), comprising an addition of a compound that “degrades,” “inactivates” and “traps and inactivates” radicals, peroxides are not sufficiently suppressed, and further technical development is needed to improve fuel cell durability.
JP Patent Publication (kokai) No. 2006-236927 (A) discloses the invention in which a membrane electrode junction for a solid polymer fuel cell having high power generation performance and capable of generating power stably for a long time contains a catalyst comprising an alloy of platinum and at least one metal selected from among cerium and manganese on a carbon carrier in the catalyst layer of either an anode or cathode. Specifically, this publication discloses a Pt/Ce catalyst and a Pt/Mn catalyst.
Similarly, JP Patent Publication (kokai) No. 2006-102568 (A) discloses a Pt/M catalyst (wherein M represents at least one member selected from a group consisting of a transition metal element, element III and rare-earth element, such as Fe, Ni, Co, Cr, Mn, Ti, Ag, Ce, La, Y and Al). JP Patent Publication (kokai) No. S-61-8851 (A) (1986) discloses a Pt/Cr—Ce alloy catalyst.
The catalysts, such as Pt/Ce catalyst, the Pt/Mn catalyst, the Pt/M catalyst and the Pt/Cr—Ce alloy catalyst disclosed in JP Patent Publication (kokai) No. 2006-236927 (A), JP Patent Publication (kokai) No. 2006-102568 (A) and JP Patent Publication (kokai) No. S-61-8851 (A) (1986) did not perform satisfactorily in terms of high current density and high durability.